Can you charge a battery with another battery?

It might be more relevant to ask how much smoke will come out of the battery.

 

That depends on several factors, including the capacitance of the capacitor. If the capacitance is too small, then the capacitor is completely charged to 12 volts and connecting a 9-volt battery to it will probably damage the battery. If its very large then the 12v battery will not charge it to even 9 volts and you can then connect the 9v battery and charge it to 9v.

See here for more info: http://www.electronics-tutorials.ws/rc/rc_1.html

 

You need higher potential to charge a battery. So if you are charging a 12V battery you would need 13-14V of any current throughput to charge that 12V bat.

 

There's some resistance and inductance in the circuit which can't be ignored when dealing with capacitors. If the inductance is negligible, then the RC product determines the charging or discharging time. With inductance, you can get damped oscillations.

 

Some years ago it was discovered that certain sub C size NiCad cells performed better in RC cars and competition aircraft after they had been "zapped". The zapping process involved charging up a bank of capacitors the size of a shoebox and discharging them into the cell using an SCR the size of a hockey puck (I heard early experiments involved using a garden spade as a switch!). The wires connecting the capacitors to the cells would twitch due to the high current and earths magnetic field. A very high percentage of the cells survived.

Do not try this at home! It's very dangerous.